It is well known to thermodynamically couple a Brayton-cycle gas turbine with a Rankine-cycle steam turbine to achieve a higher overall power plant efficiency than is obtainable for either cycle working alone (operating in the same range of working fluid temperatures and pressures). Thermal energy which would be rejected from a simple cycle Brayton gas turbine plant and lost is utilized in combined cycle configurations to heat feedwater or steam for the Rankine-cycle steam turbine and thus achieve more power generation for a given calorific input. The higher efficiency reduces fuel requirements, costs, and the quantity of undesirable effluents from the discharge of combustion products to the environment. Since most fuels for combustion power plants come from depletable fossil reserves, higher efficiencies are also desired for conservation.
One limitation on the use of integrated, high-efficiency combined cycle plants, however, results from the purity requirements of the hot gases passing through the gas turbines to avoid hot corrosion, fouling, and rapid deterioration of turbine parts. To avoid these problems, there has been a reliance on combustion of "clean fuels" such as natural gas or refined or specially treated petroleum fuels, which, unfortunately are becoming less competitive in availability and price.
Various suggestions have been made to obtain the benefits of using a fuel such as coal as the source of energy in a combined steam and gas turbine power plant since, in areas such as the United States, indigenous coal reserves are much larger than those of oil or natural gas. In one such power plant proposal, provision is made for burning coal in a pressurized fluidized bed and taking the hot combustion gas off as motive fluid to the gas turbine. Although quite efficient, this scheme necessitates a costly hot gas cleanup system and special cladding of the gas turbine hot gas path parts to prevent corrosion. To the extent that these measures are deficient or of limited life, overall power plant reliability and availability are reduced.
Willyoung, in U.S. Pat. No. 4,116,005, has disclosed one means by which sulfur-bearing coal may be burned in a fluidized bed combustor in an environmentally acceptable manner and in a way that avoids gas turbine reliability problems due to hot gas corrosion and fouling. There is taught the use of a fluidized bed combustor wherein particulate coal is burned under nearly atmospheric pressure conditions and in the presence of sulfur-sorbing particles to control emissions of sulfur oxide compounds. A stream of clean pressurized air serves as motive fluid for the gas turbine, gaining heat energy from the fluidized bed via an in-combustor heat exchanger through which the air stream passes. Steam for the steam turbine is generated also by in-combustor heat exchangers with the combination thus providing a power plant of high efficiency and capable of utilizing the energy of coal combustion in an environmentally acceptable manner without costly cleanup.
While the atmospheric fluidized bed combustor for combined cycle power plants is obviously highly advantageous in that corrosive combustion products do not pass through the gas turbine, certain limitations have nevertheless been recognized and improvements sought therefore. One limitation has been the operating temperature of the fluidized bed. To maintain the effectiveness of the sulfur-sorbing particles in removing sulfur oxides it has been necessary to keep the temperature of the bed in which the particles reside at a temperature lower than the state-of-the-art turbine inlet temperature capabilities of a modern gas turbine. This results in an overall plant efficiency somwhat less than would be obtained if higher temperature motive fluid were attainable. It has also been recognized that the greatest practical reduction in the concentration levels of oxides of sulfur and nitrogen in effluent streams is desirable, and that more compact equipment would be advantageous.
Accordingly, an object of the present invention is to provide an improved combined cycle power plant utilizing a coal fueled pressurized fluidized bed combustor wherein the sensible heat energy and motive power available from the pressurized hot combustion gas discharged from the combustor may be utilized without an extensive, costly, gas cleanup system and without need for costly, corrosion-resistant alloys or special protective cladding of the gas turbine hot gas flow path.
Another object of the invention is to provide a combined cycle power plant having a fluidized bed combustor wherein the fluidized bed is operable at temperatures higher than attainable with atmospheric fluidized bed combustors so that overall efficiency of the plant is increased and emissions of sulfur oxide are minimized.
Yet another object is to provide a more compact arrangement of combined cycle power plant equipment; one in which the bed depth of the fluidized bed combustor is optimized for heat transfer and in which formation of noxious oxide compounds is minimized.